Rolling elements

ABSTRACT

The pitting resistance of a gear is increased by hardening its tooth flanks through application of carburizing/quenching, bright hardening and induction hardening to a steel material capable of providing significantly improved softening resistance in tempering at a low temperature of 300 to 350° C. To this end, the steel material prepared so as to satisfy the relationship described by: 5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt %)) is carburized such that the carbon concentration of its carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the steel material is subjected to quenching and tempering at 300° C. or less subsequently to the carburization process, or alternatively the steel material is once cooled after the carburization process and then subjected to treatments of re-heating hardening and tempering at 300° C. or less so that a hardness of HRC 58 or more is ensured by the tempering process at 300° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional application of U.S. application Ser.No. 10/641,362, filed on Aug. 13, 2003.

TECHNICAL FIELD

The present invention relates to rolling elements produced bycarburizing and quenching, bright hardening or induction hardening. Moreparticularly, the invention relates to gears and rolling elements suchas bearings, races and rollers, the gears being made from steel whichprovides significantly improved resistance to softening caused inlow-temperature tempering at 300 to 350° C. and having high pittingresistance in the tooth flanks hardened by carburizing/quenching, brighthardening or induction hardening.

BACKGROUND ART

Up to now, gears produced by applying carburizing/quenching,carburizing/carbonitriding/quenching to SCr-based, SCM-based orSNCM-based low carbon steel have been commonly employed in the reducersof construction machines and earth-moving machines, since high contactfatigue strength (200 kgf/mm² or more) is considered to be an importantfactor. For ring gears used under the condition of comparatively lowinterface pressure (up to 150 kgf/mm²), gears produced by applyingthermal treatment such as bright hardening or induction hardening tocarbon steel or SMn-based middle carbon steel (0.45 to 0.6 wt % C) areused.

For the reducers of construction machines and earth-moving machines,less expensive gears having higher strength and higher resistance tointerface pressure are required, in view of the recent tendency tohigher output power and compactness.

Construction machines and earth-moving machines often stride obstaclessuch as rocks and structures during travelling and drill the obstacleswhile making a turn, and therefore, the gears of the reducer used forrunning and turning such machines receive impulsive load. This is aserious problem of damage to carburized quenched gears.

Bright-hardened or induction-hardened gears have higher toughness thancarburized quenched gears, but are more likely to cause pitting orscuffing when they are used under high interface pressure such as notedabove.

The invention is directed to overcoming the problem of the conventionalcarburized, quenched gears and induction-hardened gears which exhibitpoor impact resistance when they have insufficient contact fatiguestrength. Taking account of the fact that the contact fatigue strengthof a gear used under a rolling/sliding contact condition highly dependson whether or not it has sufficient temper softening resistance againstan increase up to 300° C. in the temperature of the tooth flanks duringoperation, the invention aims to provide various types of rollingelements such as carburizing and quenching gears for use under highinterface pressure, which elements are made from a steel material towhich a large amount of Al and/or Si (Al and Si can effectively increaseresistance to softening caused by tempering at 300° C.) has been addedand which elements have a temper hardness of HRC 58 or more aftertempering at 300° C. The invention further aims to provide, throughproper combined additions of Al and Ni to the above steel material,rolling elements which can exhibit high toughness in spite of their highhardness.

The present invention has been directed to overcoming the poor pittingresistance of gears hardened by bright hardening or induction hardeningand therefore aims to provide inexpensive rolling elements such as highinduction hardened gears which have been improved in temper softeningresistance so as to have a temper hardness of HRC 54 or more at 300° C.and pitting resistance equivalent to that of carburized quenched gears,by more proper additions of Si, Al, V, Mn, Cr, Mo and Ni.

DISCLOSURE OF THE INVENTION

SNCM815, SCM420, SCr420, SMnB420 steels which had been subjected tocarburizing and quenching were preliminarily tested in terms of rollingcontact fatigue strength (pitting resistance) under the condition ofrolling/sliding at interface pressures of 375 to 220 kgf/mm². As aresult, it was found that the interface pressure at which pittingappeared after 107 rotations was 210 kgf/mm² and the X-ray half valuewidth of the martensitic phase of the outermost layer of the rollingcontact surface in which pitting occurred under each pressure wasreduced to 4 to 4.2°, and significant softening was observed at theoutermost layer of the rolling contact surface.

An S55C carbon steel which has been subjected to quenching and temperingso as to have HRC 61 to 62 was preliminarily tested in terms of rollingcontact fatigue strength at an interface pressure of 250 kgf/mm². As aresult, it was found that the interface pressure at which pittingappeared after 10⁷ rotations was about 180 kgf/mm² and the X-ray halfvalue width of the martensitic phase of the rolling contact surface inwhich pitting occurred under an interface pressure of 250 kgf/mm² wasreduced to 3.6 to 4.2 similarly to the above-described carburized,case-hardened steels.

A preliminary test was also conducted on an eutectoid carbon steel (0.77wt % C) to check its rolling contact fatigue strength. As a result, itwas found that the interface pressure at which pitting appeared after107 rotations was about 230 to 240 kgf/mm² which was substantially thesame as the rolling contact fatigue strength of the aforesaidcarburized, case-hardened steels having substantially the same carboncontent. It was also found that a decrease due to variation in rollingcontact fatigue strength was observed in the carburized case-hardenedsteels because of the presence of an intergranular oxidation layer and aslack quenching layer in the rolling contact surface.

A preliminary test was conducted on an eutectoid carbon steel (0.82 wt %C), whose rolling contact surface had been subjected to inductionhardening, to check its rolling contact fatigue strength and it wasfound that the interface pressure at which pitting appeared after 107rotations was about 260 to 270 kgf/mm² and this eutectoid carbon steelhad higher rolling contact fatigue strength than the former eutectoidsteel (0.77 wt % C) because of fine cementite particles dispersing inthe martensitic phase of the rolling contact surface.

From the viewpoint of the dispersion of fine cementite particles, SUJ2containing about 1.0 wt % C and 1.5 wt % Cr was quenched at 840° C. andthen tempered to have HRC 62.5. The rolling contact fatigue strength ofthis steel was checked by a preliminary test and it was found that theinterface pressure at which pitting appeared after 10⁷ rotations wasabout 270 kgf/mm² which was approximately the same as that of the aboveeutectoid steel and that the X-ray half value width of the martensiticphase of the rolling surface in which pitting occurred under aninterface pressure of 250 kgf/mm² was reduced to 4.2 to 4.50 similarlyto the carburized, case-hardened steels described above.

Further, carbon steels having a carbon content of 0.46, 0.55, 0.66, 0.77and 0.85 wt % respectively were quenched from a temperature of 820° C.and tempered at 100 to 350° C. for 3 hours. Then, the hardness and X-rayhalf value width of each steel were checked. It was found from the testresult and studies using, as a reference, published data on these steels(e.g., “Materials” issued by Society of Materials Science, Japan, Vol.26, No. 280, P26) that the hardness when the X-ray half value width ofthe martensitic phase is 4 to 4.20 corresponds to a temper hardness ofabout HRC 51 to 53. Taking account of the fact that the surface carbonconcentrations of the carburized, case-hardened steels were adjusted toabout 0.7 to 0.9 wt %, the tempering temperature was found to be about300° C.

It is obvious from the preliminary tests described above that theoutermost surface of a tooth flank is tempered and softened by heatgenerated at the time when the gears come into engagement under highinterface pressure so that pitting occurs, and that a 300° C. -temperhardness of HRC 53 or more is necessary, as an index, for obtaining thesame level of pitting resistance as that of the carburized quenchedgears.

It has also been understood from the comparison between the 300°C.-temper hardness of the carburization-hardened layer of the SCM420steel which has undergone carburizing/quenching and the 300° C.-temperhardness of the eutectoid carbon steel which has undergone quenchingthat since virtually no improvement in temper softening resistance canbe attained by additions of Cr and Mo, a new alloy design intended forincreasing temper softening resistance during low-temperature temperingat about 300° C. is necessary in order to achieve pitting resistanceequal to or more than that of the carburized, quenched gears by brighthardening or induction hardening. Also, dispersion of fine cementiteparticles or the like in the martensitic phase has proved effective asseen from the cases of eutectoid carbon steel (0.82 wt % C) and SUJ2which were improved in rolling contact fatigue strength.

As a gear design value which provides pitting resistance equal to orhigher than the pitting resistance obtained by the carburizing/quenching(interface pressure Pmax=230 kgf/mm² or more) described above, thehardness which can withstand fatigue caused by pulsating shear stress(R=0) which is 0.3 times the value of interface pressure may be setbased on the theoretical analysis of Hertz's contact pressure. Itscalculated value is approximately HRC 53.4 which coincides with thehardness (HRC=53) obtained from the X-ray half value width of themartensitic phase of the rolling contact surface in which occurrence ofpitting was observed in the above-described preliminary test. Sincepitting occurs at the time when the temperature of the outermost portionof the rolling contact surface increases to about 300° C. owing to thefriction heat generated by rolling/sliding, it has been found that ahighly pressure-resistant gear having interface pressure resistanceequal to or higher than that of the carburized quenched gears can bedeveloped by setting 300° C.-temper hardness to HRC 54 or more which canwithstand Pmax=230 kgf/mm².

As will be described in Example 2, the 300° C.-temper hardness of themartensitic phase of a carbon steel containing 0.1 to 1.0 wt % carbon isdescribed by:HRC=36×{square root}{square root over ( )}C(wt %)+20.9

After checking, based on the above hardness, the influences of variousalloy elements upon the hardness of the martensitic phase aftertempering at 300° C., it has become apparent that the hardness of themartensitic phase after tempering at 300° C. is represented by:HRC=(36×{square root}{square root over (C)}(wt %)+20.9)+4.3×Si (wt%)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt %)×(0.45÷C (wt%))

It should be noted that the coefficient (in the case of Si for instance,this coefficient is 4.3 ΔHRC/wt %) proportional to the weight percent ofeach alloy element of the above equation indicates the temper softeningresistance of the alloy element.

In the invention, the content (wt %) of each alloy element constitutingthe above steels is defined as follows based on the above-described gearmaterials and thermal treatment designs.

To sum up, there is provided a rolling element according to theinvention which is made from a steel material containing at least 0.15to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si or 0.35 to1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); and furthercontaining one or more alloy elements selected from the group consistingof Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidableimpurities such as P, S, N and O, and balance essentially consisting ofFe; the steel material being prepared so as to satisfy the relationshipdescribed by:5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt%)×(0.45÷C (wt %)), and

-   -   which is formed by carburizing the steel material such that the        carbon concentration of a carburized surface layer of the steel        material is adjusted to 0.6 to 0.9 wt %; quenching the steel        material subsequently to the carburization process and then        tempering the steel material at 300° C.- or less, or        alternatively cooling the steel material once after the        carburization process and then applying treatments of re-heating        hardening and tempering at 300° C. or less to the steel material        so that a hardness of HRC 58 or more, more preferably, HRC 60 or        more is ensured by the tempering process at 300° C.

While the 300° C. temper-hardness of the carburized layer of anSCM-based carburized, quenched material is usually within the range offrom HRC 53 to HRC 56, the 300° C.-temper hardness is set to HRC 58 ormore in the invention on the ground that: (i) an improvement in pittingresistance can be clearly observed and (ii) taking account of the factthat the percentage of compactness when a mechanical reduction gear isdownsized by one lank is 25 to 30% and the contact fatigue strength ofthe gear in this case is no less than 1.15 times the contact fatiguestrength of the conventional gear (230→265 kgf/mm²), the 300° C.-temperhardness is HRC 58 or more.

An addition of about 1 wt % Al is apparently preferable for improvementof pitting resistance, because 15 to 25% by volume of the residualaustenitic phase existing, for example, in the carburized layer of anSCM-based steel is reduced to 10% by volume or less, which has theeffect of increasing the hardness of the carburized layer of the surface(ΔHRC=2).

It is also apparently preferable for rolling elements such as gearshaving higher strength to apply mechanical pressurization treatment suchas shot peening or roller burnishing to the tooth flanks, dedenda andtooth bottoms of the gear with the intention of improving the strengthof the tooth flanks and the bending strength of the dedenda, whereby adistinct compressive residual stress is generated. It is apparent thatthe elements to which such treatment is applied are also within thescope of the invention.

The above-described carburization is usually carried out at 900° C. ormore. Where Si and Al are contained in high concentration as describedearlier, the dual-phase (α+γ) state develops in a reheating condition inthe raw material composition part having low carbon content andpositioned deeper than the carburized layer, and quenching starts fromthis condition so that the strength of the inside of the carburizedlayer decreases. This drawback can be overcome by setting carburizedcase depth taking account of the distribution of bearing stress and thedistribution of bending stress. In addition, it is economicallydisadvantageous to increase carburized case depth and therefore, the A3transformation temperature is adjusted by adding the austenitestabilizing elements C, Mn and Ni in combination with the ferritestabilizing elements Si and Al as described earlier, thereby controllingthe temperature of carburization to a typical carburization temperatureof 950° C. or less.

In the case of a steel material containing either 1.0 to 3.0 wt % Si or0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al) toincrease temper softening resistance, an addition of 3 wt % Si increasesthe A3 transformation temperature by about 170 degrees (see FIG. 1) andan addition of 1.5 wt % Al also increases it to the same extent, wherethe carbon content is 0.20 wt %. Therefore, the upper limits of theamounts of Si and Al are set to 3.0 wt % and 1.5 wt %, respectively.According to a third aspect of the invention, Mn and/or Ni is added inthe range of 1.0 to 2.5 wt % (Mn+Ni) with the intention of restrainingthe temperature of quenching by lowering the A3 transformationtemperature through proper additions of the austenite stabilizingelements such as C, Mn, Ni and Cu.

Since carbon and nitrogen are extremely effective as an austenitestabilizing element (see FIG. 1), the lower limit of the original carboncontent of the steel material is preferably 0.15 wt % from the aboveviewpoint and the upper limit of the carbon content is preferably 0.35wt % with which the hardness of the raw material composition part insidethe carburized layer after quenching and tempering does not exceed HRC55. More preferably, the lower limit of the carbon content is 0.2 wt %.

In addition, since nitrogen often reduces the temper softeningresistance of Al, it is necessary to prevent penetration of nitride fromthe carburized or carbonitrided gas atmosphere and, therefore, creationof Al nitrides. In view of this, the N content of the carburized layeris set to at least 0.1 wt %.

According to the invention, there is provided a rolling element which ismade from a steel material containing at least 0.15 to 0.35 wt % C;further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al oralternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one ormore alloy elements selected from the group consisting of Mn, Ni, Cr,Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities suchas P, S, N and O, and balance essentially consisting of Fe; the steelmaterial being prepared so as to satisfy the relationship described by:5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt%)×(0.45÷C (wt %)), and

-   -   which is formed by carburizing the steel material such that the        carbon concentration of a carburized surface layer of the steel        material is adjusted to 0.9 to 1.5 wt %; applying treatments of        reheating hardening and tempering at 300° C. or less to the        steel material after cooling to a temperature equal to or lower        than Al temperature from a state where no cementite precipitates        in the surface layer during the carburization process, so that        fine cementite particles having a size of 1 μm or less are        dispersed within the tempered martensitic phase of the        carburized surface layer and a hardness of HRC 60 or more, more        preferably, HRC 62 or more is ensured by the tempering process        at 300° C.

The reason why the lower limit of the carbon content of the surface areaof the carburized layer is set to 0.9 wt % is that eutectoid carbonconcentration is markedly decreased by additions of Si and Cr andundissolved cementite is stably formed in an amount of 3% by volume ormore when the carbon content is 0.9 wt % or more. The reason why thecarbon content is limited to 1.5 wt % or less is that if the carboncontent exceeds 1.5 wt %, coarse cementite particles (3 μm or more) willbe unavoidably created owing to aggregation of cenentite particles, sothat there arises the high risk of a decrease in the bending strength ofthe gear. In addition, for carrying out carburization with a high carbonconcentration of 1.5 wt % or more without causing precipitation ofcoarse cementite particles in the surface layer during the carburizationprocess, it is necessary to increase carburization temperature to about1100° C. which is practically difficult because of the limitation interms of equipment.

Since the carburization which provides a surface carbon concentration of0.9 to 1.5 wt % is carried out in a high carbon potential condition witha carbon activity (ac) of about 1 and is preferably carried out in thehigh temperature region (1000° C. or more), there is the possibility ofprecipitation of coarse cementite particles in the surface layer duringthe carburization, and therefore, carbon potential needs to becontrolled with high accuracy. However, it is extremely difficult tocontrol high carbon potential carburization carried out at a temperatureof 1000° C. or more. Focussing on the fact that Cr contained in a steelmaterial promotes precipitation of coarse cementite particles, theinvention is arranged such that precipitation of cementite is preventedeven in high carbon potential carburization, by reducing the amount ofCr to 0.5 wt % or less or by limiting the amount of Cr to no more than1.4 times the amount of Si.

More precisely speaking, the effects of Mn, Ni, Mo and the like shouldbe taken into account and it is preferable to consider the relationshipdescribed by:−0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo(wt %)+0.133×V (wt %)≦0

Practically, it is preferable to set the amount of Si or (Si+Al) to 1.5to 2.5 wt % and to limit the amount of Cr to 2.0 wt % or less.

As discussed earlier, the invention is intended for dispersion of finecementite particles on condition that reheating hardening treatment isapplied to the steel and, therefore, the temperature of the reheatinghardening treatment is equal to or more than the Al transformationtemperature. In the case where Si and Al are contained in highconcentration, the (α+γ) dual phase state develops under the reheatingcondition within the raw material composition part having low carboncontent and positioned deeper than the carburized layer and quenchingstarts from this condition so that the strength of the inside of thecarburized layer decreases as described earlier, however, it is apparentthat this problem can be solved by setting the carburized case depthtaking account of the distribution of bearing stress and thedistribution of bending stress. Since increasing of the carburized casedepth is disadvantageous in view of cost, it is preferable to adjust theA3 transformation temperature by controlling the amounts of carbon, Mnand Ni in compliance with the amounts of Si and Al so that thetemperature of the reheating hardening process is set to 950° C. orless.

Where the temperature of the reheating hardening process is set to avalue as high as 850 to 950° C., it is difficult to fine cementiteparticles dispersing in an ordinary SCM-based material (0.75 wt % Mn, 1wt % Cr, 0.15 wt % Mo) so as to have a size of 1 μm or less. Therefore,the steel of the invention contains at least 0.4 wt % or less V whichcondenses to a significant degree within cementite when austenite andcementite are in the equilibrium state. It should be noted that thedistribution coefficient of each alloy element defined by a distributioncoefficient KM (KM=the concentration of M element in cementite (wt%)÷the concentration of M element (wt %) in austenite) is given by:KV=12.3, KCr=6.4, KMo=3.5, KNi=0.22, KSi, Al≈0

According to the invention, there is provided a rolling element which ismade from a steel material containing at least 0.35 to 0.60 wt % C;further containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al oralternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one ormore alloy elements selected from the group consisting of Mn, Ni, Cr,Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable impurities suchas P, S, N and O, and balance essentially consisting of Fe; the steelmaterial being prepared so as to satisfy the relationship described by:5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt%)×(0.45≈C (wt %)), and

-   -   which is formed by tempering the steel material at 300° C. or        less after quenching treatment such as induction hardening so        that a hardness of HRC 55 or more is ensured for a hardened        surface layer of the steel material by the tempering process at        300° C.

An essential condition of the invention is that the hardness of thesteel after tempering at 300° C. subsequent to quenching is HRC 55 ormore. To satisfy this condition, the steel material is preferablyhardened by quenching so as to have a hardness of about HRC 58 or moreand therefore the lower limit of the carbon content is set toapproximately 0.35 wt %. Where the temper softening resistance whichcorresponds to a 300° C.-temper hardness of HRC 53 is ensured by soleadditions of Si or Al, it becomes necessary to add 2.5 wt % or more Sior 1.47 wt % or more Al as seen from the foregoing equation, whichcauses the temperature of quenching to be as high as 900° C. or more(see FIG. 1). It is obviously more preferable for the invention toproperly control the amount of carbon which is an extremely effectiveaustenite stabilizing element, thereby restricting an increase inquenching temperature and to set the amount of carbon to 0.43 wt % ormore in order to ensure stable hardness after quenching.

A preferable upper limit of the amount of carbon is 0.6 wt % or lesswhen taking account of quenching crack susceptibility at the time ofinduction hardening. It is understood from simple calculation that wherecarbon is added within the range of from 0.4 wt % to 0.6 wt %, properamounts of Si and Al in the case of sole addition are 1.0 wt % or moreand 0.6 wt % or more, respectively.

Further, it is apparent that, in order to obtain the substantially samelevel of contact fatigue strength as the average tooth flank strength ofthe carburized, quenched gears, the 300° C.-temper hardness ispreferably HRC 55 or more, and the upper limit of the amount of carbonis preferably 0.55 wt % when taking account of susceptibility toquenching cracks caused by water or an aqueous quenching liquid used inthe induction hardening.

As an effective way of producing a gear member capable of withstandinghigh interface pressure, the above-described high carbon-concentrationcarburization is applied to the gear material of the invention such thatfine cementite particles having a size of 1 μm or less are dispersed inthe surface layer. For preventing quenching cracks occurring during thereheating hardening process or the induction hardening process, it ispreferable to utilize an aqueous quenching liquid or quenching oilhaving a high concentration of a polymer element.

To attain higher contact fatigue strength than that of theabove-described carburized quenched gears, the inventors have developeda rolling element such as a gear which is made from a steel materialcontaining at least 0.60 to 1.50 wt % C; further containing either 1.0to 3.0 wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt %(Si+Al); and further containing one or more alloy elements selected fromthe group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf,and Ca, unavoidable impurities such as P, S, N and O, and balanceessentially consisting of Fe; the steel material being prepared so as tosatisfy the relationship described by:5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt%)×(0.45÷C (wt %)), and

-   -   which is formed by tempering the steel material at 300° C. or        less after quenching treatment such as induction hardening so        that a hardness of HRC 58 or more is ensured for a hardened        surface layer of the steel material by the tempering process at        300° C.

Since there is no need to dissolve all the cementite in the austenite,the heating temperature of the induction hardening process can be set toa value in the dual phase (austenite and cementite) coexisting regionhaving an Al transformation temperature of 950° C. or less and theconcentration of carbon dissolved in the austenite can be set to asmaller value under this condition. By this arrangement and utilizationof a quenching oil or an aqueous polymer quenching liquid as a quenchingmedium, quenching crack susceptibility can be reduced.

Where fine cementite particles are dispersed by high frequency heatingand quenching, undissolved cementite is unlikely to be coarsened becausethe dispersion is carried out in a short time (within several minutes)by rapid heating, and, therefore, the addition of V is notindispensable. However, the addition of V is useful from the viewpointof further fining the structure prior to the induction hardening processand the additions of Cr, V, Mo and Mn is useful from the same viewpoint.

In addition, in the case of a rolling element such as a gear used underhigher interface pressure, it can be assumed that the rolling contactsurface is exposed to higher temperature, and V exhibits remarkabletemper softening resistance A HRC (350° C.:4.6, 400° C.:6.1, 450°C.:9.2). Therefore, the amount of V is set to 0.05 wt % or more withwhich the effect of V becomes conspicuous. The upper limit of the amountof V is set to 0.4 wt % for the reason that the effect of V can beeffectively utilized when it is added in this amount in the case wherethe maximum quenching temperature is 950° C.

Although it is favorable to positively add Mo, because Mo exertsremarkable temper softening resistance (350° C.:2.4, 400° C.:3.23, 450°C.:4.9) in the higher temperature region, the upper limit of the amountof Mo is set to 0.35 wt % from the economical viewpoint.

Where the additions of large amounts of Si and Al cause graphiteprecipitation in the process of manufacturing the steel material or inthe thermal treatment of the invention, there is a likelihood of asignificant decrease in strength. Therefore, it is apparently preferablefor the invention to add 0.2 to 0.5 wt % Cr which significantlystabilizes at least cementite and prevents graphitization.

The rolling element such as a gear contains one or more elementsselected from 0.3 to 1.5 wt % Mn, up to 0.35 wt % Mo and 0.0005 to 0.005wt % B, on the ground that the induction hardened steel material is notrequired to have high hardenability since the induction hardeningtreatment is a heating process in which heat is applied to only the partto be quench-hardened and its neighborhood, unlike furnace heating.

The present inventors have already reported in Japanese PatentApplication No. 2002-135274 that considerably high toughness can beachieved by the coexistence of Al in the aforesaid amount and 0.3 to 2.5wt % Ni and excellent Charpy impact characteristic can be obtained byit, particularly, in high-hardness martensitic structures containing 0.6wt % carbon and 1.2 wt % carbon. Ni contributes to a dramaticimprovement in the impact load resistance of a gear and is thereforeapparently beneficial as a gear material. Since the addition of Niincreases the cost of the steel material, the amount of Ni to be addedis limited to 1.5 wt % or less in the invention.

If the carburization temperature, reheating temperature and inductionhardening temperature become too high in the invention, there may arisethe problem of coarsening austenite crystal grains. In this case, it isobviously desirable to add the known elements called “crystal grainfining elements” such as Ti, Nb, Zr, Ta and Hf in an amount ranging from0.005 to 0.2 wt %.

There will be hereinafter summarized the function of each of the alloyelements constituting the above-described rolling elements of theinvention.

Si: 0.8 to 3.0 wt %

Si is an element which significantly enhances temper softeningresistance in tempering at a low temperature of 300° C. to 350° C. Themechanism of enhancing temper softening resistance is such thatsoftening is prevented by further stabilizing ε carbides whichprecipitate at low temperature and causing cementite precipitation tooccur in a higher temperature region.

(1) Induction Hardened Gears

Since the softening resistance ΔHRC of Si per 1 wt % in tempering at300° C. is 4.3 and the 300° C.-temper base hardness obtained from 0.6 wt% carbon is HRC 48.8, the amount of Si for ensuring a 300° C.-temperhardness of HRC 53 is about 1.0 wt % and the amount of Si when itcoexists with 0.15 wt % Al is about 0.8 wt %. On this ground, the lowerlimit of the amount of Si is set to 0.8 wt % and more preferably to 1.5wt % to enhance its function.

While the upper limit of the amount of Si is set to 3.0 wt % in orderthat the Ac3 transformation temperature does not exceed 900° C. and thequenching temperature is prevented from increasing more than isnecessary where the amount of carbon is within the aforesaid range offrom 0.35 wt % to 0.6 wt %, the upper limit of the amount of Si ispreferably 2.5 wt % or less in the case where the lower limit of theamount of carbon contained in the steel material for induction hardenedgears is 0.4 wt %.

(2) Carburized and Quenched Gears

The range of Si content noted earlier can be substantially suitablyapplied to the upper and lower limits of the amount of Si to be addedfor ensuring a 300° C.-temper hardness of HRC 60 in the process in whichcarburization is applied to the surface of the rolling element such as agear to increase its surface carbon content to 0.6 to 0.9 wt % and then,quenching and tempering at 300° C. or less are carried out.

Also, the range of Si content noted earlier can be substantiallysuitably applied to the upper and lower limits of the amount of Si addedfor ensuring at least a 300° C.-temper hardness of HRC 62 or more in theprocess in which carburization is applied to the surface of the rollingelement such as a gear, thereby increasing the surface carbon content to0.0.9 to 1.5 wt % and then, reheating hardening treatment is applied soas to disperse fine cementite particles in the rolling contact surface,followed by tempering at 300° C. or less.

To increase the surface carbon content of the rolling element to 0.9 to1.5 wt % without precipitating cementite in the surface layer during thecarburization process, the carbon activity in the carburization processcarried out at a high temperature of 930 to 1100° C. needs to beincreased. In this case, precipitation of coarse cementite (3 to 15 μm)(excessive carburization) mainly due to the addition of Cr element islikely to occur, resulting in a considerable decrease in the strength ofthe gear. In the invention, this problem is solved by positively addingSi which prevents excessive carburization while the amount of Cr islimited so as not to exceed 1.4 times the amount of Si, and, moreprecisely, by using a steel material which meets the relationshipdescribed by−0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo(wt %)+0.133×V (wt %)≦0

Where the above steel material is used, the vacuum carburization methodwherein carburization is carried out with a carbon activity of 1 can beemployed. This method is extremely beneficial for producing a rollingelement such as a gear, because high temperature carburization at 1100°C. or less can be carried out at low cost, and its function ofinhibiting coarse cementite precipitation is advantageous for increasingthe strength of the rolling element such as a gear.

Since Al has the strong deoxidization function as well as the strongfunction of expelling P and S from the grain boundary, P and S beingimpurities contained in steel, Al is useful for cleaning steelmaterials. Further, it has been confirmed that Al enhances tempersoftening resistance more than Si does (ΔHRC=7.3) in low temperaturetempering. In the light of the above facts, the invention is designedsuch that where Al is solely added, the amount of Al is 0.35 to 1.5 wt %and where part of Si is replaced with 0.15 to 1.5 wt % Al, the totalamount of Si and Al is 0.5 to 3.0 wt %. Since Al is a stronger ferritestabilizing element than Si as noted earlier and has the function ofincreasing the Ac3 temperature about 1.6 times higher than Si does, themaximum amount of Al is set to 1.5 wt % (=2.5 wt %/1.6) or less.

It has been reported in Japanese Patent Application No. 2002-135274 thatremarkable toughness can be achieved by the coexistence of the aboveamount of Al and 0.3 to 2.5 wt % Ni, and excellent Charpy impactcharacteristic can be achieved, particularly, in high-hardnessmartensitic structures containing 0.6 wt % carbon and 1.2 wt % carbon.Ni contributes to a remarkable improvement in the impact load resistanceof a gear and is therefore apparently beneficial as a gear material.Since the addition of Ni increases the cost of the steel material, theamount of Ni is limited to 1.5 wt % or less in the invention.

Since Mn not only exhibits remarkable desulfurization but alsostabilizes austenite as noted earlier and further has the beneficialeffect of improving the hardenability of steel, Mn is added in a properamount according to purposes. Taking account of the fact that, in asteel containing 0.35 to 0.6 wt % carbon, austenite is satisfactorilystabilized by carbon, the lower limit of the amount of Mn is set to 0.3wt %.

Mo is a useful element as it improves the hardenability of steel andrestrains temper brittleness, and therefore it is desirable for theinvention to add Mo in an amount of 0.35 wt % or less which is at thesame level as that of ordinary case-hardened steels.

In cases where the tooth flanks of a gear are quench-hardened byinduction hardening, only the surface layer which has been heated to atemperature equal to or higher than the Ac3 transformation temperatureby high-frequency heating may be quench-hardened, and therefore the gearmaterial is not required to have hardenability (DI value) higher thanthe hardenability (3.0 inches) of the ordinary carbon steel level. Thismeans that inexpensive steel materials can be employed. In theinvention, the addition of Mn and Cr is further suppressed and theaddition of alloy elements such as Si, Al, Ni, Mo and V is controlled toobtain a DI value of 3.0 inches or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram showing the effects of various alloy elementswhich constitute Fe3Si.

FIGS. 2(a) and 2(b) illustrate a test specimen used for a roller pittingtest.

FIG. 3 is a graph showing the result of a preliminary test for checkingroller pitting resistance.

FIG. 4 is a graph showing the comparison between the measured values andcalculated values of temper hardness.

FIG. 5 is a graph (1) showing the pitting resistance of steels accordingto the invention.

FIGS. 6(a) and 6(b) show patterns for vacuum carburization quenchingtreatment.

FIG. 7 is a graph (2) showing the pitting resistance of steels accordingto the invention.

FIG. 8 is a graph (3) showing the pitting resistance of steels accordingto the invention.

FIGS. 9(a) and 9(b) are photographs of the metallographic structures ofthe carburized layers of test specimens No. 5 and No. G2.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, rolling elements of theinvention will be hereinafter described according to preferredembodiments of the invention.

EXAMPLE 1 The Pitting Resistance of Quenched, Tempered Carbon Steel andCarburized, Quenched, Case-Hardened Steel

(Preliminary Test)

In this example, a roller pitting test was conducted with the testspecimen shown in FIG. 2 and the pitting resistance of various quenched,tempered carbon steels and carburized, quenched, case-hardened steelswas checked to investigate the rolling contact fatigue strength of thetooth flanks of gears. Table 1 shows the chemical compositions of thevarious carbon steels and case-hardened steels used in this example.These steel materials were respectively shaped into the small rollertest specimen shown in FIG. 2(a) and the test specimens No. 1, 2 and 4were subjected to water quenching after heating at 820° C. for 30minutes, and then tempered at 160° C. for 3 hours, followed by testing.The specimen No. 3 was quench-hardened, at its rolling contact surface,using a 40 kHz high-frequency power source after thermal refining andthen subjected to tempering similarly to the above specimens. No. 5 wascooled to 850° C. after carburization (carbon potential=0.8) at 930° C.for 5 hours. Then, it was kept at 850° C. for 30 minutes and quenched bya quenching oil having a temperature of 60° C., followed by the sametempering treatment as described above. TABLE 1 C Si Mn Ni Cr Mo NOTENo.1 0.55 0.23 0.71 S55C No.2 0.77 0.21 0.74 EUTECTOID CARBON STEEL (1)No.3 0.85 0.22 0.81 EUTECTOID CARBON STEEL (2) No.4 0.98 0.27 0.48 1.47SUJ2 No.5 0.19 0.22 0.75 0.97 0.15 SCM420H

The large roller test specimen shown in FIG. 2(b) was prepared byapplying water quenching to the SUJ2 material of No. 4 after heating at820° C. for 30 minutes and then tempering it at 160° C. for 3 hours. Theroller pitting test was carried out in such a way that the small andlarge (loaded) rollers were rotated at speeds of 1050 rpm and 292 rpmrespectively, while being lubricated with #30 engine oil having atemperature of 70° C., and a load is imposed on the rollers with a slipratio of 40% and interface pressures ranging from 375 to 220 kgf/mm².

FIG. 3 collectively shows the number of repetitions which causesoccurrence of pitting under each interface pressure. In FIG. 3, alifetime line (indicated by solid line) is shown, which is formed byconnecting the respective minimum numbers of repetitions of thereference carburized case-hardened steel tested under the variousinterface pressures. If the interface pressure when the number ofrepetitions which causes occurrence of pitting is 10⁷ times was definedas rolling contact fatigue strength, the pitting resistance was found tobe about 210 kgf/mm². When checking the pitting resistance of each testspecimen in the same way, it was found that No. 1=175 kgf/mm², No. 2=240kgf/mm², No. 3=260 kgf/mm², and No. 4=260 kgf/mm². It was further foundthat the pitting resistance of the carburized case-hardened steelsvaried to a somewhat large extent because of intergranular oxidationwhich had occurred during the carburization of the rolling contactsurface, the presence of a slack quenched layer, and a large amount ofresidual austenite. It was found from the comparison in terms of theaverage number of repetitions which causes pitting that the pittingstrengths of the carburized case-hardened steels do not differ from thatof the test specimen No. 2.

The X-ray half value width of the martensitic phase of the rollingcontact surface of each test specimen in which pitting had occurredunder an interface pressure of 250 kgf/mm² was checked. As a result, itwas found that No. 1=3.6 to 4.0°, No. 2=4 to 4.2°, No.3=4.2 to 4.4°,No.4=4.3 to 4.6° and No.5=4 to 4.2°.

Further, the test specimens Nos. 1 to 5 which had undergone theabove-described thermal treatment were tempered at 250 to 350° C. for 3hours and then, the X-ray half value width of the rolling contactsurface of each test specimen in which pitting had occurred was checked.As a result, the half value width of each specimen under the abovecondition was found to be coincident with the half value width whentempering was carried out at 300° C. It was also found to besubstantially coincident with the relationship between the temperhardnesses and half value widths of carbon steels having various carbonconcentrations which was reported in “Material” Vol. 26, No. 280, P26.

EXAMPLE 2 Checking of Temper Softening Resistance

Table 2 shows the alloy compositions employed in this example. Thermaltreatment was carried out in such a way that after heated at 810 to 870°C. for 30 minutes, each test specimen was subjected to water cooling andthen tempering at 300° C. or 350° C. for 3 hours. Thereafter, theRockwell hardness HRC of each test specimen was checked and the effectof the addition of each alloy element on the hardness was analyzed.TABLE 2 TPNo. C Si Al Mn Ni Cr Mo V B No.6 0.45 1.45 0.46 1.49 0.52 0.140.0018 No.7 0.49 1.45 0.46 1.01 1.03 0.15 0.0019 No.8 0.47 0.31 0.462.01 1.03 0.15 0.0019 No.9 0.49 0.29 0.45 1.5 1.49 0.23 0.0019 No.100.36 1.77 0.6 0.62 0.11 0.0026 No.11 0.45 0.95 0.68 0.01 1.29 0.5 0.0029No.12 0.39 0.93 1.02 0.08 0.97 0.95 0.5 No.13 0.43 0.26 0.44 1.01 0.480.001 No.14 0.47 0.25 0.4 1.01 1.05 0.0018 No.15 0.46 1.5 0.4 1 0.510.002 No.16 0.45 0.24 0.4 1.02 0.48 0.31 0.0011 No.17 0.45 1.46 0.390.96 0.98 0.001 No.18 0.41 0.25 0.35 1 0.49 0.0017 No.19 0.52 2.3 0.570.11 No.20 0.98 0.27 0.48 1.47 No.21 0.55 0.23 0.71 No.22 0.77 0.21 0.74No.23 0.45 0.21 1.26 0.53 1.51 0.21 No.24 0.6 0.25 0.97 0.93 0.98 1.040.35

In a preliminary experiment, the hardness of a carbon steel containing0.1 to 1.0 wt % carbon and 0.3 to 0.9 wt % Mn was checked to be utilizedas base data for the analysis of the effect of each alloy element. As aresult, it was found that the hardness of this steel was approximated bythe following equations.HRC=34×{square root}{square root over ( )}C (wt %)+26.5 (temperingtemperature=250° C.)HRC=36{square root}{square root over ( )}C (wt %)+20.9 (temperingtemperature=300° C.)HRC=38×{square root}{square root over ( )}C (wt %)+15.3 (temperingtemperature=350° C.)

After analyzing the effect of each alloy element based on the hardnessesof the carbon steels noted above, it was found that the temper softeningresistance ΔHRC in the case of tempering at 300° C. for instance couldbe described by the following equation.ΔHRC=4.3×Si (wt %)+7.3×Al (wt %)+1.2×Cr (wt %)×(0.45÷C(wt %))+1.5×Mo (wt%)+3.1×V (wt %)

It was found from this result that Al exerted temper softeningresistance 1.7 times higher than that of Si and was therefore extremelyeffective as an element for improving rolling contact strength.

FIG. 4 shows the degree of coincidence of the temper hardness obtainedfrom the result of the above analysis with the temper hardness obtainedfrom actual measurements. It will be understood from FIG. 4 that temperhardness can be accurately estimated with the variation range of HRC±1.The 300° C. -temper hardness of the carburized layer (0.8 wt % carbon)of SCM420 (No. 5) of Example 1 is indicated by mark ⋆ in FIG. 4 and wellcoincident with the calculated value.

EXAMPLE 3 An Improvement in Pitting Resistance by Use of Steel MaterialsHaving Excellent Temper Softening Resistance 1

Table 3 shows the alloy components of the steel materials used in thisexample. The test specimens No. P1 to No. P10 were subjected totempering at 160° C. for 3 hours subsequently to quenching at 850 to920° C., whereas the test specimens No. 11 and No. 12 were subjected toinduction hardening under the same high frequency heating condition asin Example 1. A roller pitting test was conducted on these testspecimens. TABLE 3 300° C. C Si Al Mn Ni Cr Mo V B calculated HRC No.P10.34 0.21 1.47 1.17 0.17 0.11 53.96 No.P2 0.39 1.49 0.49 0.51 0.34 0.0553.91 No.P3 0.41 1.51 0.72 0.32 0.15 51.09 No.P4 0.41 1.5 0.71 0.32 0.160.3 51.99 No.P5 0.45 0.18 1.26 0.53 0.5 0.21 55.94 No.P6 0.55 1.51 0.710.15 0.16 54.48 No.P7 0.61 1.21 0.75 0.14 54.34 No.P8 0.62 0.21 1.240.53 0.12 59.31 No.P9 0.45 1.02 1.26 0.49 0.12 58.78 No.P10 0.61 0.251.47 0.93 0.98 1.04 0.35 62.27 No.P11 0.83 1.01 0.31 0.55 0.96 0.3862.11 No.P12 1.21 0.2 0.52 0.52 1.01 0.51 0.4 66.62

The test for checking pitting resistance was carried out undersubstantially the same condition as in Example 1 and the test result isshown in FIG. 5. In FIG. 5, the pitting occurrence line obtained inExample 1 is indicated by solid line and the pitting occurrence lineobtained in Example 3 is indicated by broken line.

It was found from the above result that the pitting resistance of therolling contact surface can be dramatically improved by the soleaddition of Al or Si or the combined addition of Al and Si and foundfrom the comparison between the test specimens No. P3, P4, P11 and P12that the pitting resistance of the rolling contact surface can bedramatically improved by the addition of V.

A remarkable improvement in pitting resistance was observed in the testspecimens No. 11 and No. 12 which had been induction hardened such thatfine cementite particles were dispersed in the martensitic phase of therolling contact surface.

Table 3 shows the 300° C. temper hardness obtained from the calculationand it will be understood that this temper hardness has good conformityto the interface pressure at which pitting occurs after 10′ timesrepetitions, the interface pressure being calculated from this temperhardness.

EXAMPLE 4 An Improvement in Pitting Resistance by Use of Steel MaterialsHaving Excellent Temper Softening Resistance 2

This example is intended to increase interface pressure strength bycarburizing and quenching treatment. Table 4 shows the alloy componentsof the test specimens. Two kinds of carburizing/quenching treatments asshown in FIGS. 6(a) and 6(b) were applied. The treatment shown in FIG.6(a) is vacuum carburization carried out at 950° C. (with the intentionof achieving a carbon concentration of 0.8 wt % in the carburized layer)with methane gas free from N2 gas and the treatment shown in FIG. 6(b)is carried out at 1,020° C. (with the intention of achieving a carbonconcentration of 1.3 wt % in the carburized layer). The test specimensfor the high-carbon concentration carburization (1.3 wt % C) weresubjected to quenching and tempering after reheating at 900° C. for 30minutes. TABLE 4 C C Si Al Mn Ni Cr Mo V B No.5 0.19 0.22 0.75 0.97 0.15No.G1 0.22 0.83 0.72 0.96 0.16 No.G2 0.21 1.48 1.18 0.45 0.21 0.41 No.G30.26 0.81 0.37 1.21 0.49 0.52 0.19 No.G4 0.26 0.21 1.01 1.09 1.51 0.150.37 No.G5 0.34 0.21 1.47 1.17 0.17 0.11 No.G6 0.22 0.24 0.99 1.29 1.011.02 0.36

A roller pitting test was made under the same condition as in Example 1.The test result is shown in FIGS. 7 and 8. FIG. 7 shows the result ofthe test using the test specimens which were subjected tocarburizing/quenching/tempering treatment intended for a carburizedsurface layer containing 0.8 wt % C. Compared with the result of thereference specimen No. 5, No. G3 to No. G6 containing Al have presentedan obvious improvement. It has been proved by the results of No. G2 andNo. G3 that the sole addition of Si definitely leads to an improvementwhere the amount of Si is approximately 1.0 wt % or more.

FIG. 8 shows the result of a pitting test conducted on the testspecimens No. 5, No. G2 and No. G4 which were subjected toreheating/quenching/tempering treatment such that the carburized surfacelayer had a carbon content of 1.3 wt % and such that cementite particleswere dispersed in the tempered martensitic phase of the rolling contactsurface. Compared with the reference specimen No. 5, they weresignificantly improved in interface pressure strength.

FIGS. 9(a) and 9(b) show structural photographs of the carburizedsurface layers of the test specimens No. 5 and No. G2. In No. G2, thecementite particles dispersing in the martensitic phase and, therefore,the martensite were fined by the addition of V, which obviouslycontributed to the significant improvement in interface pressurestrength.

1. A rolling element which is made from a steel material containing atleast 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Sior 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al); andfurther containing one or more alloy elements selected from the groupconsisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca,unavoidable impurities such as P, S, N and O, and balance essentiallyconsisting of Fe; said steel material being prepared so as to satisfythe relationship described by:5≦4.3×Si (wt %)+7.3×Al (wt %)+3.1×V (wt %)+1.5×Mo (wt %)+1.2×Cr (wt%)×(0.45÷C (wt %)), and which is formed by carburizing said steelmaterial such that the carbon concentration of a carburized surfacelayer of the steel material is adjusted to 0.6 to 0.9 wt %; quenchingthe steel material subsequently to the carburization process and thentempering the steel material at 300° C. or less, or alternativelycooling the steel material once after the carburization process and thenapplying treatments of re-heating hardening and tempering at 300° C. orless to the steel material so that a hardness of HRC 58 or more isensured by the tempering process at 300° C.
 2. The rolling elementaccording to claim 1, wherein, in said steel material, the amount of Cris limited to no more than 1.4 times the amount of Si, and one or moreelements selected from the group consisting of 0.35 wt % or less Mo, 0.4wt % or less V, 1.0 to 2.5 wt % (Mn+Ni) are added in an amount whichsatisfies the relationship described by:−0.146×Si (wt %)+0.03×Mn (wt %)−0.024×Ni (wt %)+0.075×Cr (wt %)+0.043×Mo(wt %)+0.133×V (wt %)≦0.
 3. The rolling element according to claim 1,wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al) andless than 2.0 wt % Cr to prevent precipitation of cementite during thecarburization process.
 4. The rolling element according to claim 1,wherein said steel material having an Al content of 0.3 wt % or morecontains 0.3 to 1.5 wt % Ni.
 5. The rolling element according to claim2, wherein said steel material contains 1.5 to 2.5 wt % Si or (Si+Al)and less than 2.0 wt % Cr to prevent precipitation of cementite duringthe carburization process.
 6. The rolling element according to claim 2,wherein said steel material having an Al content of 0.3 wt % or morecontains 0.3 to 1.5 wt % Ni.